An axial flow compressor comprises a plurality of rotor blades extending radially from a rotary hub, an outer casing surrounding the rotor blades defining a certain gap between the opposing tips of the rotor blades and the inner circumferential surface of the outer casing, and a plurality of stator blades extending radially inwardly from the inner circumferential surface of the outer casing immediately downstream of the rotor blades. See Japanese patent laid-open publication No. 11-200808.
In such an axial flow compressor, the kinetic energy of the rotor blades is converted into the kinetic energy of the fluid, thereby accelerating the fluid, and the stator blades located downstream of the rotor blades change the direction of the fluid flow, thereby decelerating and increasing the static pressure of the fluid. Therefore, to improve the efficiency of an axial flow compressor, it is necessary for the stator blades to be able to change the direction of the fluid flow with a minimum loss.
As the rotor blades rotate at a high speed and the tip of each blade moves along the inner circumferential surface of the outer casing defining a small gap between them, a highly complex flow pattern is generated near the tip of the rotor blade owing to the development of a surface boundary layer, generation of a shockwave, leaking of flow across the tip of the rotor blade and interferences of these flows. In particular, the interference between the leak flow produced between the tip of the rotor blade and the opposing inner circumferential surface of the outer casing and the shockwave produced between adjacent rotor blades produces a low momentum region having a certain circumferential expanse behind a rear half of each rotor blade (see FIG. 10). Furthermore, a trailing flow extending along the blade surface and having a limited circumferential expanse is generated at the outlet end of the rotor blade owing to the development of a surface boundary layer and occurrence of flow separation (see FIG. 11).
Because such a low momentum region and/or a trailing flow are produced cyclically, the flow between adjacent stator blades is prevented from becoming steady. In other words, to improve the efficiency of an axial flow compressor, it is necessary for the stator blades to be designed by taking into account not only the steady-state flow at the exit end of the rotor blades but also the non-steady-state flow in this region. Such a non-steady-state flow is a cause of noises.